A free rotational speed estimation device is known (for example, Japanese Patent No. 3412363) that calculates a vehicle body speed and respective free rotational speeds of vehicle wheels of a vehicle (“free rotational speed” as used here refers to the rotational speed of each vehicle wheel when no braking force is applied). The calculated results of this free rotational speed estimation device are utilized in a vehicle behavior control apparatus that stabilizes vehicle behavior by controlling the braking force of each vehicle wheel. With this known free rotational speed estimation device, the free rotational speed of each vehicle wheel is estimated based upon (i) respective vehicle wheel speeds when a braking force control apparatus is not applying braking force to the vehicle wheels, or (ii) the vehicle body speed at that time.
In normal vehicles, such as two-wheel or four-wheel drive vehicles, drive torque is transmitted to a left and right pair of drive wheels, respectively, via a differential. The key characteristics of such general-use differentials are as follows. (1) When respective vertical loads of the left and right drive wheels is different, drive torque of the drive wheel with greater vertical load is partially transferred to the drive wheel with the smaller vertical load by the differential. Accordingly, the drive torque of the drive wheel with smaller vertical load is increased. (2) When there is a difference in the braking force applied to the left and right drive wheels, drive torque of the drive wheel subject to the larger braking force is partially transferred to the drive wheel subject to the smaller braking force. Accordingly, the drive torque of the drive wheel subject to smaller braking force is increased.
When high lateral acceleration is acting on a vehicle during turning, the vertical load of the drive wheel at the outside of the turning direction (hereinafter referred to as the “outside wheel”) becomes lower than the vertical load of the other drive wheel at the inside of the turning direction (hereinafter referred to as the “inside wheel”). However, given the characteristics of the differential described above, drive torque of the outside wheel with the larger vertical load can be partially transferred to the inside wheel with the smaller vertical load, whereby the drive torque of the inside wheel is increased. In addition, on some occasions, in order to perform vehicle behavior control, braking force is applied to the outside wheel among the pair of drive wheels when high lateral acceleration is acting in the above described manner. In this case, the characteristics of the differential cause even more of the drive torque from the outside wheel to be transferred to the inside wheel. As a result, the drive torque of the inside wheel is increased substantially.
When the drive torque of the inside wheel of the pair of left and right drive wheels is increased in this way, acceleration slip occurs, whereby a rotational speed of the inside wheel (i.e., a vehicle wheel speed) that is detected by a vehicle wheel speed detection unit increases. This vehicle wheel speed, in the form of a sensor output, is larger than an actual vehicle body speed at the inside wheel position. Therefore, a vehicle body speed estimated based on the vehicle wheel speed of the inside wheel in this manner is larger than the actual vehicle body speed. In other words, a slip ratio of the vehicle wheel speed of the inside wheel increases.
However, in the related art, the fact that the vehicle wheel speed increases due to increase in the drive torque of the inside wheel when high lateral acceleration is acting is not taken into consideration. Accordingly, the vehicle wheel speed and the vehicle body speed that are obtained are not accurate. When vehicle behavior control is perform based upon these inaccurate parameters, there are occasions when the resultant vehicle behavior stabilization effect is inadequate.